With four years of nearly-continuous photometry from Kepler, we are finally in a good position to apply asteroseismology to $gamma$ Doradus stars. In particular several analyses have demonstrated the possibility to detect non-uniform period spacings, which have been predicted to be directly related to rotation. In the present work, we define a new seismic diagnostic for rotation in $gamma$ Doradus stars that are too rapidly rotating to present rotational splittings. Based on the non uniformity of their period spacings, we define the observable $Sigma$ as the slope of the period spacing when plotted as a function of period. We provide a one-to-one relation between this observable $Sigma$ and the internal rotation, which applies widely in the instability strip of $gamma$ Doradus stars. We apply the diagnostic to a handful of stars observed by Kepler. Thanks to g-modes in $gamma$ Doradus stars, we are now able to determine the internal rotation of stars on the lower main sequence, which is still not possible for Sun-like stars.
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